The present invention generally concerns so-called stents for angioplasty.
The term xe2x80x9cstentxe2x80x9d is intended generally to indicate devices intended for endoluminal application (for example, within a blood vessel) usually effected by catheterisation, with the subsequent expansion in place for the local support of the lumen. The principal aim of this is to avoid the re-establishment of a stenotic site at the treated site. It should be noted that the use of substantially similar structures to effect the expansion and anchorage in place of vascular grafts has already been proposed in the art: naturally, this possible extension of the field of application should also be seen as being included within the ambit of the invention.
For a general review of vascular stents, reference may usefully be made to the work xe2x80x9cTextbook of Interventional Cardiologyxe2x80x9d edited by Eric J. Topol, W. B. Saunders Company, 1994 and, in particular, to section IV of volume II, entitled xe2x80x9cCoronary stentingxe2x80x9d.
Many patent documents have addressed this problem, such as, for example, U.S. Pat. No. 4,776,337, U.S. Pat. No. 4,800,882, U.S. Pat. No. 4,907,336, U.S. Pat. No. 4,886,062, U.S. Pat. No. 4,830,003, U.S. Pat. No. 4,856,516, U.S. Pat. No. 4,768,507, and U.S. Pat. No. 4,503,569.
One problem that is not yet completely resolved in connection with the implantation of a stent is in relation to restenosis which, depending on the type of lumen in question, may be more or less likely to occur. Several studies have shown that the principal mechanism causing restenosis after the stent-implantation operation is a hyperplasia of the neointima mediated by the cells of the smooth muscle.
It has, however, been noted that nuclear radiation, in particular xcex2-radiation, inhibits the formation of the neointima. The manufacture of a stent capable of emitting nuclear radiation has therefore already been proposed: in this way, after implantation, the surrounding tissues become irradiated, which inhibits the above-mentioned hyperplasia.
To this end, atoms of the P32 radionuclide are injected, by means of a cyclotron, onto the surface of a stent made from conventional material, for example, stainless steel, before it is implanted (see xe2x80x9cRadioactive Stents for the Prevention of Neointimal Hyperplasiaxe2x80x9d by Tim A. Fischell, from xe2x80x9cEndoluminal Stentingxe2x80x9d chapter 18, p. 134 (1996) edited by W. B. Saunders Company Ltd).
According to a similar technique (see xe2x80x9cTechnical and Engineering Aspects of Stents Which May Be Either Permanent or Removablexe2x80x9d by R. Makkar et al., from xe2x80x9cEndoluminal Stentingxe2x80x9d, chapter 32, p. 230, (1996) edited by W. B. Saunders Company Ltd), a titanium stent is bombarded with protons having an energy equal to 8 MeV, which provoke the reaction Ti48(p,n)V48, also leading in this case to the formation of a radioactive nuclide.
These methods require the use of very complicated equipment, such as a cyclotron, for accelerating the charged particles. In addition, since these particles are retained on the surface layer of the stent body, a sophisticated system must be provided for moving the latter in order to expose as much as possible of its surface to the particle beam.
Overall, therefore, these known techniques, although valid for research, are not suited to the mass production of radioactive stents.
The object of the present invention is that of overcoming the aforesaid disadvantages, and the invention has the characteristics referred to specifically in the claims.
In one aspect, the invention is a stent for angioplasty having a body in the form of a generally tubular casing capable of being dilated in use from a radially-contracted position to a radially-expanded position, the body comprising a support structure of a first material capable of withstanding dilation without losing its structural integrity, and having associated with at least a part of the support structure a structure made from a second material which is made radioactive following the exposure of the stent to a neutron flux.
The structure made from the second material may comprise a continuous layer on at least a portion of the support structure or on the entire support structure. The continuous layer may range in thickness between 0.4 and 1 micrometer.
In a preferred embodiment, the structure made from the second material forms a core within the support structure. The core may have a diameter of between 10 and 100 micrometers.
In another preferred embodiment, the structure made from the second material is in the form of a plurality of inserts housed in associated recesses formed on the surface of the support structure.
The second material may be iridium, tantalum or mixtures thereof.
In another aspect, this invention is a method for the production of a stent having a body in the form of a generally tubular casing capable of being dilated in use from a radially-contracted position to a radially-expanded position, the body comprising a support structure of a first material capable of withstanding dilation without losing its structural integrity, and having associated with at least a part of the support structure a structure made from a second material which is radioactivatable, wherein the body is exposed to a neutron flux so as to render the second material radioactive.
The body may further comprise a carbon film.
The association of the second structure with the support structure may be achieved by means of galvanic techniques or sputtering, by coextrusion, or by welding, mounting or inclusion.
In yet another aspect, this invention is an intermediate product obtainable by the method for the production of a stent having a body in the form of a generally tubular casing capable of being dilated in use from a radially-contracted position to a radially-expanded position, the body comprising a support structure of a first material capable of withstanding dilation without losing its structural integrity, and having associated with at least a part of the support structure a structure made from a second material which is radioactivatable, wherein the body is exposed to a neutron flux so as to render the second material radioactive.